The product of the invention includes a pattern and, following processing, is useable as a disposable sheet, (table) napkin or other decorative item.
This kind of web is manufactured by a known procedure wherein paper pulps are dry-defibered, a web is formed on a forming canvas wherein the individualized fibers are randomly distributed aeraulically, a thermoplastic binder is introduced to penetrate the web so formed to allow the fibers to interlace, followed by drying and the carrying out of crosslinking.
Where desired, a pattern can be impressed by calendering and/or embossing the web before the binding stage to thereby impart a different binder distribution. This binder then preferentially diffuses into the compacted zones.
Dry-deposited cellulose fibers are known which are composed of randomly distributed short cellulosic fibers such as wood pulp and of a thermoplastic binder, and which are provided with a pattern on at least one side of the sheet. At their surfaces, such webs include compressed and uncompressed zones. The pattern can be implemented by embossing or by marking the sheet.
Herein, the expression "embossing" denotes creating, on part of the sheet, surface protrusions with corresponding recesses on the remaining sheet surface. In other words, an embossed sheet has gained thickness.
The expression "marking" herein denotes compressing given portions of one sheet surface by forming compacted zones and thereby reducing sheet thickness in these zones without thereby creating protrusions on the opposite side. The compressed or marked zones have higher density than the uncompressed zones. The unmarked zones may or may not be calendered.
Marking of dry-deposited sheets has been used to improve binder application to the sheets. Illustratively, in U.S. Pat. No. 4,127,637, part of the sheet surface is compressed by cylindrical rollers prior to the binding stage with the pattern then being binder stabilized. As regards U.S. Pat. No. 4,135,024, one sheet surface is marked while simultaneously a binder is applied to the other sheet surface which thereby improves binder penetration into the sheet. In this kind of procedure, web marking represents a stage within the very manufacturing process of the web.
In European Patent Document No. 0,077,005, the dry-deposited fiber web is marked to increase its bulk and to improve its absorbency. At least 40%, and preferably 50% to 80% of the total area of one sheet surface is marked. Both sides may be marked. The procedure consists in marking the dry-deposited sheet with a recess-pattern using cylinder rollers preferably lacking resilience and being heated to 140.degree. C. to 180.degree. C. and applying sufficient pressure to raise the density of the web portions situated underneath the compressed zones and to achieve a density of about 0.2 to 0.5 g/cm.sup.3. The marking stage is carried out anytime after the dry fibers have been deposited on a moving support and after the web has been strengthened.
Nevertheless, while the absorbency in this product has been improved, it also evinces a strength, in particular rupture resistance, tending to decrease because of the shears applied to the fibers. Surface properties such as feel and velvety nature are degraded. Very likely, the marking procedure employed for this kind of product and the substantial size of the marked surface, that is of the compacted zones, are at the root of the degradation of properties. The product is harsher and less comfortable to use and, as a result, its applicability suffers in some instances, such as in use as napkins.
Moreover, being dry-manufactured, the fiber distributions of the webs are hardly uniform. Some web portions have fiber accumulations whereas others have light zones with far fewer fibers. When marking by means of an engraved steel cylinder and a slave cylinder also made of steel, there will be "super-thick" and well-marked high-density zones whereas the low-density zones are poorly marked or not marked at all. In the marked web, the high-density zones subtend "wedges" between which marking is slight. It is impossible to uniformly spread the pressure over its full generatrix. Therefore marking will be irregular. The web so marked lacks a uniform pattern and will be inferior aesthetically. This defect is present in dry-deposited webs which, in particular, have a density dispersion of .+-.20%.
U.S. Pat. No. 4,476,078 mentions this problem in marking following web drying and crosslinking. The description observes that in using this process, there may be neither good marking definition nor high grade pattern printing. The Patent offers as a solution a process in which the binder following its application to the web will be partially crosslinked in order to achieve a deforming and partially crosslinked sheet which is physically stable while being moved to the marking station, and during marking proper, further is marked and, lastly, completion of crosslinking of the marked sheet. Such a procedure entails drawbacks, namely, marking must be at the crosslinking station and very likely therefore at the very site of web manufacture, and the procedure per se is longer and more complex.
Most of the above prior art patents propose marking during the very manufacture of the web. As a result, severe difficulties are encountered in quickly and economically exchanging one engraved cylinder for another with a different pattern. Such procedures lack adaptability to market requirements.